1. Field of the Invention
The present invention relates to a displacement detection apparatus that reduces an accumulated position error of a scale used for the displacement detection apparatus.
2. Description of the Related Art
An encoder (a displacement detection apparatus) is a device that reads periodic marks contained previously to calculate a relative displacement amount based on a product of a mark pitch and the number of the mark pitches. Because the accuracy of the mark pitch is converted into a displacement error directly, it is important to reduce a size error of the mark pitch in order to improve the accuracy of the encoder. However, even if the size error of the mark pitch is reduced, a scale recording the marks may be obliquely mounted on the apparatus and also the mark pitch may be expanded or contracted due to the mechanical stress or the temperature change in fixing and mounting the scale. Therefore, it is difficult to improve an accumulated position accuracy of the encoder.
Previously, the accuracy has been improved by performing a correction processing for a displacement measurement value that is measured by the encoder. For example, Japanese Patent Laid-Open No. H8-63597 discloses an accumulated error correction method that performs an accumulation calculation of a displacement of a measured value of a scale that is to be corrected with reference to a measured value of a highly-accurate standard scale to correct a scale pitch by a resolution of a detection apparatus when the accumulation amount is beyond a predetermined value. Japanese Patent Laid-Open No. 2006-345254 discloses a detection velocity measurement method that corrects velocity information containing an error of a mark pitch based on two position detection sensors at a distance of a known interval from each other.
However, in the configurations disclosed in Japanese Patent Laid-Open No. H8-63597 and Japanese Patent Laid-Open No. 2006-345254, it is necessary to previously prepare a highly-accurate standard scale in order to correct the accumulated position error of the mark pitch of the scale used for the displacement detection apparatus. In addition, it is necessary to configure the displacement detection apparatus which includes the highly-accurate standard scale along with this apparatus in order to estimate a mark pitch error or an accumulated position error value of the scale in a state of being mounted on the apparatus. In other words, it is necessary to correct the apparatus itself using the standard scale. In particular, when the scale is made of an inexpensive thin-film material, the linearity of the scale cannot be maintained and an amount of the mark pitch error is enlarged. This is caused by the scale meandering in a movable direction or expanding and contracting in accordance with an amount of tension for example. Furthermore, the mark pitch may always vary since the scale expands and contracts depending on the environment temperature change in accordance with the thermal capacity of the material. Accordingly, it is generally necessary to use the scale on which the mark pitch is recorded with high accuracy by using a material capable of suppressing the mark pitch error to the minimum.